Frobisher wants to die, Sixsmith refuses to let him go.

John has always claimed to be "not gay" and for a while, Sherlock didn't care. Then he started having feelings that he was unfamiliar with for his flat mate. Love is a confusing concept. It complicates friendships, breaks hearts, and gives joy more powerful than any other. Basically, love complicates everything, especially for Sherlock Holmes.